1. Field of the Invention
This invention relates to variable-gain amplifiers having very high dynamic range and low distortion. More particularly, this invention relates to such amplifiers where the gain can be varied exactly exponentially by an analog control signal, for example, to provide a swept-gain amplifier for use in ultrasound imaging equipment having medical applications.
2. Description of the Prior Art
Variable-gain amplifiers have been used for many years, and a wide variety of techniques have been employed for controlling the gain. The subject is so central to the topic of signal processing that the literature is too extensive to summarize with any brevity. However, it does appear that one common theme in many analog variable-gain techniques is the employment of non-linear circuit elements directly in the signal path, e.g., use of the exponential relationship between collector current and base-emitter voltage in a bipolar junction transmitter. All such non-linear schemes, however, have relatively poor accuracy, in particular, residual non-linearity, which often is caused by non-ideal aspects of transistor behavior and the necessity for an open-loop signal path.